Rsync Special Files – Understanding Rsync’s Relationship with Special Files

Rsync with --iconv would be an option, but you'd first need to define an encoding where ?, : and others are encoded with characters that are allowed on FAT filesystems.

For your use case, you aren't using all the power of rsync for this. This task can be done in a few lines of shell script.

Here's a bash script that copies ~/Music to /media/usb99, skipping files that are older on the target, and converts : and ? to _. It doesn't detect clashes (I assume you don't have both foo:bar.mp3 and foo?bar.mp3).

#!/bin/bash
set -e
shopt -s dotglob globstar
cd ~/Music
for source in **/*; do
  target=/media/usb99/${source#"$HOME"/}
  target=${target//[:?]/_}
  if [[ -d $source ]]; then
    mkdir -p -- "$target"
  elif [[ $target -ot $source ]]; then
    cp -p -- "$source" "$target"
  fi
done

This script works under zsh with minor modifications: replace shopt -s dotglob globstar by setopt dot_glob and [[ $target -ot $source ]] by [[ ! -e $target || $target -ot $source ]].

rsync -a --no-specials --no-devices would tell rsync to skip these files. It will still print an information message, but it would return 0 if no other error occurs.

If there's a set of known paths that you don't want to transfer, you could exclude them altogether. Also, do pass the -x option to skip all mounted filesystems (including /dev, which takes care of the biggest offender), and if there are multiple on-disk filesystems, list all the mount points (e.g. rsync -ax / /home /destination).

rsync -ax --exclude='/var/spool/postfix/dev/*' / /mnt/shared/backup

If none of that is satisfactory, make a list of files you want to skip. Beware that if some of the file names are under control of an adversary, they could cause some files to be omitted from a backup. For example, if they create a directory whose name is a newline and create a named socket called * inside it, then using the output of find -type s as an exclude list would result in /* being excluded. To prevent such problems, keep problematic names out of the exclude list.

{
  cd /path/to/source &&
  find . -name '*[\[?*
]*' -prune -o \
               \( -type b -o -type c -o -type p -o -type s \) -print |
  sed 's/^\.//'
} | rsync -a --exclude-from=- /path/to/source /destination